1. Field of the Invention
The present exemplary embodiment relates to a conductive polymer suspension and a method for producing the same, a conductive polymer material, and a solid electrolytic capacitor and a method for producing the same.
2. Description of the Related Art
Conductive organic materials are used for the electrodes of capacitors, the electrodes of dye-sensitized solar cells and the like, the electrodes of electroluminescent displays, and the like. Conductive polymers obtained by polymerizing pyrrole, thiophene, aniline, and the like are known as such conductive organic materials.
Such a conductive polymer is generally provided as a dispersion (suspension) or a solution in an aqueous solvent, or a solution in an organic solvent, and is used as a conductive polymer material by removing the solvent in use. But, even if the type of the conductive polymer is the same, the physical properties of the obtained conductive polymer material are different depending on the state of the dispersion, and therefore, various studies have been made on the method for producing the dispersion.
JP7-90060A discloses techniques regarding a solution (dispersion) of polythiophene and a method for producing the same, and its use in the antistatic treatment of a plastic molded article. This dispersion of polythiophene includes water or a mixture of a water-miscible organic solvent and water as a dispersion medium, polythiophene composed of the structural unit of 3,4-dialkoxythiophene, and a polyanion derived from polystyrenesulfonic acid having a molecular weight in the range of 2,000 to 500,000. The polythiophene is obtained by chemical oxidative polymerization in the presence of a polyanion of polystyrenesulfonic acid having a molecular weight in the range of 2,000 to 500,000. It is said that thus, a transparent antistatic film can be formed.
JP2004-59666A discloses techniques regarding a water dispersion of a composite of poly(3,4-dialkoxythiophene) and a polyanion and a method for producing the same, and a coating composition containing the water dispersion, and a coated substrate including a transparent conductive film formed by coating with the composition. This water dispersion is obtained by polymerizing 3,4-dialkoxythiophene in an aqueous solvent in the presence of a polyanion, using peroxodisulfuric acid as an oxidant. Alternatively, this water dispersion is obtained by subjecting 3,4-dialkoxythiophene to chemical oxidative polymerization in an aqueous solvent in the presence of a polyanion, using an oxidant, with the pH of the reaction solution decreased by adding an acid selected from the group consisting of water-soluble inorganic acids and organic acids. It is said that thus, a conductive thin film having excellent transparency can be formed.
JP2007-119631A discloses a method for producing conductive polymer fine particles dispersed in an organic solvent. According to this, it is said that conductive polymer fine particles dispersed in an organic solvent can be produced by performing electrolytic polymerization, with ultrasonic irradiation, in an emulsion obtained by mixing and stirring a monomer, an organic solvent, water, and a surfactant, and then removing the water layer.
JP2007-297500A discloses a technique regarding conductive polymer fine particles dispersed in water. According to this, it is said that conductive polymer fine particles stably dispersed in water are obtained by subjecting a monomer to chemical oxidative polymerization in water in the presence of a reactive emulsifier, and that the particle surface is coated with the reactive emulsifier or copolymerized with the reactive emulsifier.
However, in a method for subjecting 3,4-dialkoxythiophene to chemical oxidative polymerization in one stage, in the presence of a polyanion acting as a dopant, as in the methods described in JP7-90060A and JP2004-59666A, the control of the doping rate is difficult. In other words, undoped polyanions, that is, polyanions not contributing to conductivity, are present in an excess amount, and it is difficult to say that this method is sufficient as a production method for obtaining a conductive polymer material having higher conductivity.
In addition, a conductive polymer film obtained by the method described in JP7-90060A has sufficient conductivity as an antistatic material, but when it is used, for example, as the solid electrolyte of a capacitor, it is difficult to achieve conductivity that sufficiently satisfy requirements for lower ESR. In other words, the surface resistivity of an antistatic film is generally 105 to 1014Ω/□, and too high conductivity may cause severe electrostatic discharge, and therefore, it is considered that the antistatic film does not have such conductivity that can quickly dissipate the static electricity of a charged object. Therefore, a material that can be used as an antistatic film has high resistance as the solid electrolyte of a capacitor and cannot meet requirements for low resistance. In addition, a capacitor including a solid electrolyte containing excess polyanions has a disadvantage that its reliability, particularly its properties in a higher humidity atmosphere, is poor.
The conductive polymer fine particles dispersed in the organic solvent, produced by the production method described in JP2007-119631A, contains the surfactant. Therefore, a conductive polymer film obtained by removing the organic solvent contains the surfactant, and a problem of the conductive polymer film is that it has high resistance.
In addition, in JP2007-119631A, the monomer is dissolved in the organic solvent or in the water, and further, the water layer is removed later, and therefore, the organic solvent and the water are substances that are not dissolved in or mixed with each other, and the surfactant is added for the purpose of emulsifying the organic solvent and the water. Therefore, JP2007-119631A discloses no technique regarding the production of a conductive polymer suspension containing no surfactant, in which a monomer is emulsified in water.
In the conductive polymer fine particles dispersed in water described in JP2007-297500A, the emulsifier is present on the particle surface. Therefore, a conductive polymer film obtained by removing the solvent contains the emulsifier at the particle interface, and a problem of the conductive polymer film is that it has high resistance.
In addition, in JP2007-297500A, the fine particles are stably dispersed in the water by containing the reactive emulsifier on the particle surface, and JP2007-297500A discloses no technique regarding the production of a conductive polymer suspension containing no surfactant.
It is an object of the present exemplary embodiment to solve the above problems and specifically to provide a conductive polymer suspension for providing a conductive polymer material having high conductivity and a method for producing the same, and particularly provide a solid electrolytic capacitor having low ESR and a method for producing the same.